Ooids Research Articles

Wave dynamics and sediment transport in Great Salt Lake

Great Salt Lake is a natural laboratory to test and refine ideas about the relationship between sediment transport by waves and the characteristics of shoreline carbonate sediments, in particular ooid sands and microbialite mounds. In this chapter, we present a year-long series of wave data collected from July 2021 through June 2022 and use these wave data to assess the performance of a US Army Corps of Engineers wave model previously used to estimate bed shear velocity and intermittency of sediment transport in Great Salt Lake (Smith and others, 2020). We use this model-data comparison to identify the strengths and weaknesses of the existing model for both geological and ecological applications, and areas of improvement for future model development. We also use shallow sediment cores and Unmanned Aerial Vehicle (UAV)-based orthomosaics collected from shorelines near each buoy to assess how the wave climate along two parts of the lake shore influences the stratigraphic record and the surface morphology of the lakebed.

Controls on bioturbation and sediment distribution in carbonate shoreface deposits: insights from heterogeneity in Pleistocene and Recent strata

ABSTRACT Burrowing organisms alter sedimentary textures, influence cement distribution, and affect petrophysical characteristics of carbonate strata. Although many descriptions of carbonate successions reference bioturbation, quantitative data on spatial variability of trace fossils is rare, and fewer studies address trace-fossil influence on postdepositional modification of sedimentary deposits, which can affect petrophysical properties. To address these unknowns and determine the controls on ichnology in carbonate shoreface successions, this study evaluates the along- and across-strike distribution of sediment and bioturbation in recent, Holocene, and Pleistocene shoreface deposits on the leeward margin of Crooked–Acklins Platform (CAP), southern Bahamas. To the north, the extant margin shelf is characterized by poorly to moderately sorted, very fine–fine, skeletal–peloid–ooid sand with an average of 16% mud (< 62.5 µm) that is moderately to intensely bioturbated (ii3–6). Trace assemblages are diverse, and include horizontal tracks and trails, abundant horizontal deposit-feeding and locomotion traces, as well as dwelling and resting burrows attributable to the proximal Cruziana Ichnofacies. In marked contrast, shelf margin deposits to the south are well-sorted, medium ooid–peloid sand with < 1% mud, and display a range of bioturbation, from nonbioturbated to moderately intense bioturbation (ii1–4). Trace-fossil assemblages exhibit low ichnodiversity, dominated by vertical dwelling burrows with reinforced wall lining attributable to the Skolithos Ichnofacies. Holocene and Pleistocene strata show similar proximal-to-distal and along-strike variations in sediment attributes, ichnodiversity, and bioturbation. These trends reflect a progressive, north-to-south increase in energy reflecting the change in margin orientation relative to the direction of dominant wave energy, analogous to the recent system. This study provides data for an integrated sedimentologic–ichnologic conceptual model for processes and patterns of sediment accumulation on carbonate shorefaces, and are distinct from siliciclastic analogs.

Fossils, pseudofossils and problematica from the Ura Formation: Ediacaran of the Patom Basin, Siberia

The pseudofossils, problematica and actual macrofossils from the Middle Ediacaran Ura Formation of the Patom Basin of Siberia are critically reviewed. Both the pseudofossils and problematica are flat-clasts of ooid grainstones that are interpreted as a product of submarine gravitational or storm-induced erosional episodic events. Some extremely flat batten-shaped ooid and cementstones 2D-clasts closely resemble shreds of ribbon-like macrophyte phytoleims randomly scattered on a bedding plane. They can potentially be considered as a cement-incrustation along the algae thalli buried within ooid sands or shales, however, there is no certainty in their taxonomic identification. At the same time, other abiotic or microbially induced formation mechanisms of these fragments cannot be excluded. The Ura macrofossils, similar in size to the 2D-clasts, are described from the same outcrop, approximately 60 m downsection. They are represented mainly by the ribbon-shaped carbonaceous compressions of Liulingjitaenia alloplecta that demonstrate the stratigraphically lowest appearance of the Miaohe-type fossils in mid-Ediacaran (pre-Shuram, 580–570 Ma) strata of Ura Formation in close association with Doushantuo-type acanthomorphic acritarchs. These fossils are present as a detrital component or as in situ buried large (decimeter scale) colonial clusters. The colonies covered significant surface areas of the soft muddy sea-floor beneath wave-base. L. alloplecta themselves, commonly attributed to green macroalgae, could also be considered as composite colonies of filamentous microalgae or cyanobacteria.

Development and collapse of the early Cambrian shallow-water carbonate factories in the Hannan-Micangshan area, South China

The development of carbonate factories has potential importance in studies of paleoenvironment, sedimentology, and reservoir geology, but the mechanisms controlling the growth and demise of the large amount of carbonate sedimentation have not been broadly investigated, especially in terms of deep-time sedimentary records. Here, we provide a high-resolution study of the development and collapse of a series of tropical shallow-water carbonate factories distributed in the Hannan-Micangshan area of the South China Craton during the early Cambrian (Age 3). Unlike modern tropical shallow-water factories, the major carbonate producers in the early Cambrian factories contained ooids, benthic microbial communities (predominantly calcified microbes), and symbiotic microbial-skeletal organisms (i.e., calcimicrobial–archaeocyath bioherms). The ooid factories were mostly distributed in the tidal sand complexes around the Hannan-Micangshan massif; and some of the mixed quartz-ooid deposits may have been remodeled by terrigenous sands sourced from the adjacent fluviodeltaic delivery. The microbial carbonate factory was primarily characterized by thrombolites in terms of the lithological composition, with a small contribution of stromatolites and oncolites (grain sizes mostly of 2–4 cm). The thrombolites formed not only mound architectures individually (length of ~1–5 m, and height of ~0.5–2.5 m), but also small- to large-scale bioherms associated with reef-building skeletal organisms (i.e., archaeocyaths). Nutrient conditions may have played an essential role in regulating the distributions of the microbial and microbial-skeletal factories (mesotrophic conditions), but they did not have a significant effect on the ooid factories. These tropical shallow-water carbonate factories were located in oxygenated aqueous environments with some small transient low-oxygen intervals that corresponded to barrier microbial and microbial-skeletal factories in the study area. Hydrodynamic condition and other controls (e.g., wind) may have played roles in shaping the factory morphology and distribution features in the study area. The chemical index of alteration (CIA; values of 60–65) and petrological features (occurrence of large-scale ooid sand complexes) support a warm and dry climate during the development of these tropical shallow-water factories. Finally, the transition from carbonate-dominated to siliciclastic-dominated depositional systems driven by wetter climate conditions (CIA, close to 70) may have been responsible for the cessation of the shallow-water carbonate factories in the Hannan-Micangshan area, which resulted in a rapid drowning, and subsequently, continuous input of a high volume of siliciclastics in late Age 3 to early Age 4 (Yanwangbian Formation). This study provides clues to understanding the growth and demise of deep-time tropical carbonate factories and serves as a valuable reference for studies of other regions and time periods. • We present a high-resolution study of the tropical shallow-water carbonate factories. • These factories contained ooids, calcified microbes, and skeletal organisms. • Nutrient conditions may have played a key role in regulating factory distributions. • Hydrodynamic condition and wind shaped the factory morphology and distribution. • The shift of the depositional system was likely responsible for factory collapse.

Non-lithifying microbial ecosystem dissolves peritidal lime sand

Microbialites accrete where environmental conditions and microbial metabolisms promote lithification, commonly through carbonate cementation. On Little Ambergris Cay, Turks and Caicos Islands, microbial mats occur widely in peritidal environments above ooid sand but do not become lithified or preserved. Sediment cores and porewater geochemistry indicated that aerobic respiration and sulfide oxidation inhibit lithification and dissolve calcium carbonate sand despite widespread aragonite precipitation from platform surface waters. Here, we report that in tidally pumped environments, microbial metabolisms can negate the effects of taphonomically-favorable seawater chemistry on carbonate mineral saturation and microbialite development.

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, China

ABSTRACT Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.

Controls of depositional facies patterns on a modern carbonate platform: Insight from hydrodynamic modeling

AbstractRecent studies of Holocene deposits on Great Bahama Bank (GBB) have focused on mapping of sediment distribution, an analysis of the variable filling of accommodation space, a comparison between ooid sandbodies and an examination of whitings and mud production. Collectively, these studies provide insight into the variability in depositional facies that challenge outcrop and subsurface interpretations. Missing from these analyses, however, has been a scrutiny of the physical controls over deposition. To partially explore these controls, a hydrodynamic model forced by prevailing ocean hydrodynamics, tides, winds and atmospheric pressure was developed. Current intensity and direction can be examined at short time steps over 1 year, and GBB can be partitioned into zones of mean annual hydrodynamic energy. Areas of vigorous tidal exchange correspond to localities where platform margin ooid sand shoals have developed. There is a predictive relationship between increasing peak current velocity and increasing area of the sandbody for the Cat, Joulter and Schooners Cays and Tongue of the Ocean sites. A connection between platform‐top hydrodynamics and the formation/suppression of whitings is evidenced west of Andros, suggesting a relationship between the production/deposition of platform‐top muds and off‐platform circulation. Filling of accommodation space can be partially related to platform topography and hydrodynamic flow. For instance, accommodation filling occurs locally along the platform margin by grainstones in areas of high tidal exchange as well as by mud accumulation leeward of islands. Conversely, the development of hiatal surfaces (sites of non‐deposition) occurs in areas not sheltered by islands, such as the southern GBB, where platform‐top currents are persistently vigorous. A broader understanding of platform‐top currents and their diverse controls can aid interpretation of the rock record, including the type and distribution of platform‐top sediments, the distribution of hiatal surfaces and, therefore, identification of locations which have the potential to host more complete depositional cycles.

Modern carbonate ooids preserve ambient aqueous REE signatures

Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.

The Formation and Distribution of Modern Ooids on Great Bahama Bank.

Great Bahama Bank (GBB) is the principal location of the formation and accumulation of ooids (concentrically coated, sand-size carbonate grains) in the world today, and as such has been the focus of studies on all aspects of ooids for more than half a century. Our view from a close look at this vast body of literature coupled with our continuing interests stresses that biological mechanisms (microbially mediated organomineralization) are very important in the formation of ooids, whereas the controlling factor for the distribution and size of ooid sand bodies is the physical energy. Mapping and coring studies of the modern ooid sand bodies on GBB provide insight into the rock record from different perspectives. An important consequence of the dual influence of ooid formation and distribution is that the geochemical signature of ooids is not in equilibrium with the seawater in which ooids form; therefore, extracting the paleophysical energy record from oolitic deposits is potentially more accurate than doing so for the paleochemical record.

Impact of strong easterly trade winds on carbonate petroleum exploration - Relationships developed from Caicos Platform, southeastern Bahamas

Brisk, persistent easterly trade wind influences define Holocene patterns of carbonate sedimentation across the Caicos Platform (southeastern Bahamas). Resultant predictive sedimentary facies models based on trade wind influences are more widely applicable to the exploration for subsurface carbonate plays than are existing models based on the northern Bahamas facies patterns, which are characterized by gentle trade winds and strong platform-margin-related oceanic processes (swells and tidal currents). The Caicos Platform relationships may be more applicable because many ancient shallow carbonate depositional environments were within the trade wind belts and commonly within broad intracratonic seas that were little influenced by oceanic processes.The grainstone-dominated Caicos Platform exhibits reservoir potential over much of its surface, in contrast to northern Bahamian platforms, where oceanic tidal currents or swells and gentle easterly trade winds confine higher energy environments with reservoir potential to platform margins. Strong easterly trade winds across Caicos Platform promote widespread Holocene platform-interior oolitic, skeletal and grapestone grainstone bodies on this platform. Orientations of ooid sand bodies vary depending on preexisting topography, water depth and bottom energy. Shallow subtidal ooid sand shoals orient parallel to these winds. Ooid sands developed along older shorelines orient parallel to the shorelines but prograde perpendicular into these winds. Deeper platform-interior oolitic sands exist as widespread, sheet-like deposits. These trade winds allow reefs and ooids to coexist in many settings, permit isolated linear reefs to flourish on certain leeward platform margins, and promote effective off-bank transport of carbonate sands that create onlapping grainstone wedges. These relationships are very applicable to the rock record.Strong trade wind influences, such as seen on Caicos Platform, better explain the occurrence of Cretaceous reef and/or oolitic grainstone reservoirs developed well in from platform margins (Fairway Field in East Texas; Black Lake Field in Louisiana) than do existing northern Bahamian models. Depositional models based on conditions in the northern Bahamian models would predict low-energy facies in these platform-interior settings. A trade-wind-driven depositional model, characterized by strong persistent easterlies, also better explains the origin of the onlapping wedge of skeletal grainstones at Poza Rica oilfield in Mexico.The Caicos Platform easterly trade-wind-driven depositional model should generally apply to any ancient shallow carbonate environment that developed 5–22° north or south of the paleoequator, whether or not a platform margin was near by. Future carbonate exploration or exploitation should always factor in not only geological age, but also physiographic, latitudinal and climatic setting at a global and local scale.

Quantitative interrogation of a fossilized carbonate sand body – The Pleistocene Miami oolite of South Florida

AbstractExposures of the Pleistocene Miami oolite in South Florida provide excellent examples of preserved primary sedimentary features and subsequent diagenetic changes of a ‘fossilized’ ooid sand body that has been subaerially exposed in a tropical climate since its deposition during the last interglacial highstand – Marine Isotope Stage 5e. Using a robust airborne light detection and ranging digital terrain model and select outcrops, a detailed analysis of the morphologies and dimensions of the different portions of the Miami oolite was conducted. The sand body, extending 95 km north to south and approximately 15 km wide, consists of shoals (or bars) separated by tidal channels and is partly bounded on the ocean‐facing side by a prograding barrier bar that collectively cover an area of approximately 1000 sq. km. Results of the quantitative interrogation of the Miami oolite are compared with those of previously published work from modern sand bodies on Great Bahama Bank including the Exumas, Schooners Cays and Tongue of the Ocean. Digital terrain models developed for these modern sand bodies on the Great Bahama Bank and the ancient Miami oolite were used as the basis for the definition of shoals, shoal crests (peaks) and channels which were examined with respect to their size, shape and orientation. The Exumas sand body is the best morphometric match when analysing the bars and the channels, and the Joulter Cays area is an analogue for the formation of the barrier bar and for burrow reworking of cross‐bedding within stabilized sand shoals. The Exumas is a particularly compelling analogue for the Miami oolite with respect to length and overall visual comparison between the morphologies of the sand bodies, shoal (or bar) shape, number of tidal channels, channel length and width, and high areas parallel to the strike trend of the sand body (islands in the Exumas and possible islands as part of the Miami oolite barrier bar). Despite ca 115 kyr of subaerial exposure and meteoric diagenesis including karst, the morphology of the Miami oolite is still relatively intact to the point where it can be easily analysed and meaningful comparisons to modern systems can be made. The Miami oolite serves as a key reference example for comparison to Holocene sand units in the Bahamas – it validates the concept of comparative sedimentology and in particular emphasizes how results from the modern can improve the interpretation of a fossilized example.

Contrasts between wave- and tide-dominated oolitic systems: Holocene of Crooked–Acklins Platform, southern Bahamas

As the stratigraphic record is neither homogenous nor isotropic, a fundamental challenge in sedimentary geology is characterizing and predicting the nature of variability. Although many nearshore to shallow-marine siliciclastic depositional systems have been grouped into wave-dominated and tide-dominated classes, many facies models of carbonate systems have not recognized or applied this distinction. To fill this gap, the purpose of this study is to compare and contrast the hydrodynamic processes, sedimentology, geomorphology, and stratigraphy of an oolitic tide-dominated shoal complex and an oolitic wave-dominated shoreline system from the same area (Crooked–Acklins Platform, southern Bahamas). Field, petrographic, granulometric, remote-sensing, and hydrodynamic data illustrate marked contrasts between the two end members. The wave-dominated shoreface system near French Wells on the leeward margin is influenced by small (Hs <30 cm) waves and only weak, wave-driven currents (<15 cm/s, net southward flow); the passage of Hurricane Irene (in 2011) had only a local impact on the shoreface. The low-energy setting favors generation and alongshore transport of ooids and peloids of fine- to medium-sized sand, and has facilitated nearly 1 km of shoreface progradation, forming geomorphic bodies with margin-parallel geometry. The stratigraphic record of the area includes a coarsening- and shoaling-upward succession. In contrast, the ~7-km-wide, tide-dominated shoal on the southwestern margin is dominated by vigorous tidal currents (in excess of 50 cm/s, and reversing twice daily). These currents generate margin-normal bars and channels consisting of oolitically coated grains of medium to coarse sand size. Stratigraphically, above a basal clast-rich deposit, sediment size and sorting vary across the shoal. The open-ocean and the platformward flanks of the shoal, most impacted by tide and wave energy, include the cleanest oolitic sand, with fines accumulating in the stabilized shoal interior. Overall, the oolitic shoal aggraded and expanded platformward over bioturbated peloid–ooid sand. These results reveal how sediment, geomorphology, and stratigraphy differ between tide- and wave-dominated end members, patterns broadly analogous to several ancient examples. These data collectively illustrate the processes that create multi-scale linkages among stratigraphic patterns, geological facies bodies, and granulometry (e.g., depositional porosity and permeability). These linkages provide information that could be used to predict sedimentological and geomorphical trends in ancient analogues.

Molecular indicators of microbial diversity in oolitic sands of Highborne Cay, Bahamas

Microbialites (stromatolites and thrombolites) are mineralized mat structures formed via the complex interactions of diverse microbial-mat communities. At Highborne Cay, in the Bahamas, the carbonate component of these features is mostly comprised of ooids. These are small, spherical to ellipsoidal grains characterized by concentric layers of calcium carbonate and organic matter and these sand-sized particles are incorporated with the aid of extra-cellular polymeric substances (EPS), into the matrix of laminated stromatolites and clotted thrombolite mats. Here, we present a comparison of the bacterial diversity within oolitic sand samples and bacterial diversity previously reported in thrombolitic and stromatolitic mats of Highborne Cay based on analysis of clone libraries of small subunit ribosomal RNA gene fragments and lipid biomarkers. The 16S-rRNA data indicate that the overall bacterial diversity within ooids is comparable to that found within thrombolites and stromatolites of Highborne Cay, and this significant overlap in taxonomic groups suggests that ooid sands may be a source for much of the bacterial diversity found in the local microbialites. Cyanobacteria were the most diverse taxonomic group detected, followed by Alphaproteobacteria, Gammaproteobacteria, Planctomyces, Deltaproteobacteria, and several other groups also found in mat structures. The distributions of intact polar lipids, the fatty acids derived from them, and bacteriohopanepolyols provide broad general support for the bacterial diversity identified through analysis of nucleic acid clone libraries.

Relations between geomorphic form and sedimentologic-stratigraphic variability: Holocene ooid sand shoal, Lily Bank, Bahamas

Although the linkages among surface sediments, geomorphic forms, and hydrodynamics in Holocene ooid tidal sand shoals have been evaluated recently, how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained. Yet, such understanding is essential to developing meaningful predictive conceptual models of three-dimensional architecture of ancient reservoir analogs. Integrating remote-sensing imagery, high-frequency seismic data, and core characterization from Lily Bank, a modern tidally dominated Bahamian ooid shoal in which sedimentologic processes are well documented, reveals the stratigraphic record of geomorphic change. An irregular, gently dipping rocky surface (interpreted as the top Pleistocene) with no pronounced topographic high underlies the Holocene oolitic succession. A 6-m (20-ft)–thick poorly sorted, gravelly muddy sand with few ooids overlies this basal surface. This lower interval is overlain by sand with an upward increase in proportion of ooids, sorting, and grain size. The uppermost unit, present only under active bar forms, is well-sorted oolitic medium sand with accretionary foresets. Sediments vary stratigraphically and geomorphically; the lower unit is finer and less well sorted than the upper units, and in the oolitic upper unit, sediment size and sorting on bar crests are distinct from bar flanks. Collectively, these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position, as well as geobody size and orientation.

Controls On Links Between Geomorphical and Surface Sedimentological Variability: Aitutaki and Maupiti Atolls, South Pacific Ocean

Abstract Isolated carbonate platforms and atolls occur throughout the geologic record, from Archean to present. Although the roles of sea-level change, tectonics, and sediment supply on large-scale changes in stratigraphical architecture of these systems are relatively well understood, the details of the nature and controls on sedimentological variability between and within individual geomorphic elements on platforms and atolls are less well constrained. The purposes of this study are (1) to describe, compare, and contrast geomorphological and sedimentological characteristics of surficial Holocene sediments from two south Pacific atolls (Aitutaki and Maupiti) representing different physical and chemical oceanographic settings, and (2) to explore possible causes for sedimentological heterogeneity by contrasting the patterns with those present on other platforms and regions. Both atolls include annular shelf-edge reef systems with a red algal rim and reef flat, flanked lagoonward by reef sand aprons up to 1,500 m wide. These aprons consist of moderately sorted coarse sand and gravel, with an abundance of Halimeda, coral, and red algal fragments; broadly, sediments become finer and better rounded away from margins. Water depths increase abruptly inboard of these reef sand aprons, into platform interiors with depths up to 25 m (Maupiti) and 9 m (Aitutaki). Although patch reefs dot these lagoons, sediment is commonly poorly sorted with bimodal and polymodal size distributions and includes a mix of whole and fragmented skeletal grains, peloids, and ooids; mud and fine sand are abundant (up to 80% on Maupiti, 85% on Aitutaki). The occurrence of peloids, fine ooid sand, and oolitically laminated skeletal grains on Maupiti Atoll (as previously documented on Aitutaki) is consistent with interpretations of the impact of elevated regional ocean total alkalinity (TA) and pH levels in this part of the Pacific. On these wave-dominated atolls, open-ocean waves impinging on the platform margins generate currents that transport sediment from the reef, across the sand apron, and towards the platform interior. Unlike the tide-dominated platforms of the Bahamas, however, weak tides preclude strong daily agitation and circulation, so ooids do not grow large, and mud and fine sand transported into the atoll interiors is sequestered there. In the interiors, both sediment produced in situ and fine allochthonous sediment are more influenced by biological processes than by physical transport processes, so bioturbated muddy sand is most common. Collectively, the comparative analysis provides actualistic, process-based conceptual models for the nature and controls on variability of shelf-margin reef and reef apron systems and platform interiors, within and among atolls.

Polygonal sandcracks: Unique sedimentary desiccation structures in Bahamian ooid grainstone

Abstract Sandcracks, which are ubiquitous in Holocene eolian and beach backshore carbonate grainstone on Alligator Point, Cat Island, Bahamas, resemble polygonal mudcracks, but formed in ooid sand without muddy matrix. In experiments on Cat Island beach sand, sediment surfaces cracked polygonally in the absence of mud or biofilms while drying at room temperature due to contraction generated by capillary effects related to surface tension attraction of interstitial water. Gravitational collapse of irregular open pores and repacking of sand grains due to loss of cohesion between particles caused by evaporation of water enhance the cracking process and appearance of polygons by providing space for cracks to expand. The polygons are held together by any remaining capillary moisture and associated meniscus cement, which precipitates as the sand dries. Polygonal sandcracks can be preserved by rapid lithification of carbonate sand, but have been documented only rarely from other localities because their formation requires well-sorted, well-rounded spherical grains rather than those making up the more common, heterogeneous skeletal and peloidal sediment in carbonate settings. Interpretation of this primary sedimentary desiccation structure provides new insights into sedimentation and diagenesis of ooid-rich deposits and can aid in recognizing ancient subaerial exposure horizons.

A New Dichotomous Microboring: Abeliella bellafurca isp. nov., Distribution, Variability and Biological Origin

Cluster-forming boring traces in carbonate substrates, characterized by regular dichotomous ramifications, are described as a new ichnospecies of Abeliella Mägdefrau 1937: A. bellafurca isp. nov. The new trace differs from A. riccioides Mägdefrau by a three- rather than two-dimensional display of tunnels and branchings and by penetrating carbonatic rather than phosphatic substrates. These boring traces occur in bivalve shells from the Oligocene to the Recent in shallow, illuminated marine settings. Two distinct morphotypes of A. bellafurca were observed, both originating from a single point of entry: one produces prostrate boring system in early developmental stages followed by deeper penetration, and the other, which forms only deep penetrating three-dimensional boring systems. Tunnels with dichotomous ramifications in modern marine environments are produced by the cyanobacterium Hyella stella Al-Thukair et Golubic. The organism penetrates the shells and shell fragments, whereby the apical cell divides alternately parallel and perpendicular to the direction of penetration. Bifurcation follows each longitudinal division of the apical cell. This type of cell division and bifurcations was described in Neoproterozoic endolithic cyanobacterium Eohyella dichotoma Green, Knoll et Swett. Both Proterozoic and modern organisms penetrated carbonatic ooid sand grains in shallow marine settings. Other species of Hyella and Solentia are known to form traces described as Fascichnus (Radtke). Dichotomously branched microborings in phosphatic substrates described as Abeliella riccioides Mägdefrau were attributed to boring fungi. Distinctions between borings produced by phototrophs vs. heterotrophs are discussed because they have bearing on paleobathymetric interpretations.

Controls on morphology and sedimentology of carbonate tidal deltas, Abacos, Bahamas

Although the physical processes controlling siliciclastic shoal bodies are reasonably well-understood, the sedimentary processes that shape carbonate tidal shoal systems are poorly constrained. The tidal deltas of the northwestern Abaco Islands, Bahamas, provide a unique opportunity to explore the relationships between physical and sedimentary processes in a variety of sizes and shapes of tidal deltas. The length of tidal deltas range in size from ~ 350 m to 2 km (from the inlet to the termination of the delta lobe) and shoal crests consist primarily of well-sorted ooid sands (mean grain size between 400 and 500 μm; McManus sorting factor ≤ 0.70). Coated grains are rare off the shoals, and sorting decreases with distance from the shoal crests. Velocities in the rocky inlets can exceed 2 m/s in the daily tidal cycles, and the systematic distribution of grain types reflects tidal velocity variations. Areas with the highest velocities (≥ 0.80 m/s) correspond to hard, rocky bottoms with some skeletal grains. These regions of elevated velocities are focused within the inlets, where jets of high velocity currents form during each tidal cycle. These current jets extend outward from the inlet throat, and velocities decrease outwards as the flow disperses. Velocities between 40 and 80 cm/s occur in the regions with abundant ooids, and peloids are most abundant in the lowest velocity regions (< 0.40 m/s) in the tidal delta system. At a larger scale, inlet tidal prism and delta size are closely correlated in these tidal deltas, as in their siliciclastic counterparts. The results of this study illustrate a clear relationship between quotidian physical processes and their geomorphic and sedimentologic products in a carbonate system. In this context, carbonates appear to share many geomorphic similarities with siliciclastic analogs. The results represent an important step beyond qualitative facies interpretations and towards quantitative, specified environmental and sedimentologic characterizations for carbonate systems.

Molybdenum isotopic composition of modern and Carboniferous carbonates

We investigate the redox-sensitive isotope system of molybdenum (Mo) in marine carbonates to evaluate their potential as archive of the Mo isotopic composition of coeval seawater. We present Mo isotope data ( δ 98/95Mo) of modern skeletal and non-skeletal carbonates as well as a variety of precipitates from the mid and late Carboniferous. The external reproducibility is determined by repeated analyses of two commercially available carbonate standards. The resulting uncertainty of the low concentration samples is ± 0.1‰ (2 σ). Analysis of modern ooid sands from the Bahamas shows a consistently heavy Mo isotopic composition ( δ 98/95Mo between 2 and 2.2‰), approaching modern mean seawater values ( δ 98/95Mo = 2.3‰ ± 0.1‰ (2 σ)). This suggests that isotope fractionation during Mo uptake into non-skeletal carbonate precipitates is small. In contrast, modern skeletal carbonates show variable isotopic compositions (0.1 to 2.2‰) which suggests a biologically controlled fractionation process. The varying Mo signatures found in Carboniferous cement phases point to a strong response to local changes in fluid composition from which they precipitated. Overall, we recognized three important factors to cause an offset relative to ocean water: Mo derived from skeletal components, input of detrital Mo and admixture of light, hydroxide derived Mo via diagenetic fluids. All of these factors cause a lighter Mo isotopic composition relative to seawater. Due to the apparent small isotope fractionation during Mo uptake into non-skeletal carbonates, their δ 98/95Mo closely reflects the ambient fluid composition. From these results we conclude that carbonates represent a promising new tool to characterize the water mass Mo isotopic composition of marine paleo-environments.

Iron ooid beds of the Carolinefjellet Formation, Spitsbergen, Norway

Iron ooid beds are unusual deposits that have been linked to greenhouse conditions and the transgressive flooding of shallow shelves, and which were globally prevalent during certain periods. Within the marine, Aptian–Albian, Carolinefjellet Formation of Spitsbergen, chamosite ooids have been found within distinctive sandstone beds at six localities, and at a consistent stratigraphic position within the basal Dalkjegla Member. Distinctive characteristics include the iron ooids themselves, a coarser grain size, intercalation with silty siderites, grading, cross-beds indicating offshore or longshore transport, and a lack of burrowing. The enclosing sands display planar and hummocky crossstratification and abundant oscillation ripple marks, and are interpreted as lagoon-attached bar complexes. The stratigraphic position and traits of the iron ooid sands are consistent with seaward storm transport and preservation within interbar swales. Ooids vary in shape considerably, and display evidence for multiple growth events. Nuclei of quartz, opaques, carbonate clasts and laminated crusts are typically encircled by finer grained tangential chamosite and opaque laminae, sometimes with outer overgrowths of calcite and/or radial chamosite. The Dalkjegla Member is the marine portion of a large-scale transgressive tract, attached to underlying fluvio-estuarine Helvetiafjellet Formation strata. A lagoonal environment associated with the basal shales of the Dalkjegla Member represents a logical setting, where riverine iron concentration and iron silicate growth could occur. The Spitsbergen iron ooid beds extend the known occurrence of Cretaceous examples, representing a less common High-Latitude example, and one not directly associated with a transgressive flooding surface.